1. Field of the Invention
The present invention relates to an intermediate branching method (e.g. mid-span branching method) for an optical path including a multi-fiber optical cable constituted by a multi-fiber ribbon with a plurality of optical fibers arranged in a plane and integrated with a common coating.
2. Related Background Art
Sheaves S of an optical cable C are installed in manholes M or near electric poles P in advance for the case of intermediate branching (see FIG. 5). Extra portions of the optical cable C are preserved, e.g., coiled, and stored as the sheaves S. Since the optical cable C has these sheaves S, an intermediate branching operation of the optical cable C is executed smoothly and efficiently.
An actual mid-span branching operation is executed by using an operation vehicle in which connection can be made by using a generator. More specifically, an operation vehicle V is parked near a manhole or an electric pole. One of the sheaves S of the optical cable C is uncoiled, and an obtained optical cable C1 is led into the vehicle. An optical cable C2 to be connected after branching has been led into the vehicle. Operations such as branching, connection, closure mounting, and the like are performed for the optical cables C1 and C2 in the vehicle. When these operations are completed, the optical cable C1 is returned in the manhole M.
As methods of branching an optical cable, a method using a chemical, an inflating method (see FIG. 2 of Japanese Patent Laid-Open No. 63-206704 as shown in FIG. 6A of this application), a planing method (see FIG. 2 of Japanese Patent Laid-Open No. 63-206705 as shown in FIG. 6B of this application), and the like are known.
The method using a chemical is not appropriate for intermediate branching since it chemically damages the common coating of the ribbon with certain chemical. Such chemical damages not only the common coating of the multi-fiber ribbon but also the coatings of the individual optical fibers. Accordingly, an optical fiber which is branched by using a chemical is highly liable to disconnection, and thus its overall long-term reliability after installation is degraded.
The inflating method and planing method can be applied to intermediate branching of an optical cable. However, they can be used only if the common coating of the ribbon to be used has a certain thickness and thus does not easily peel off. Also, the common coating must be removed. Therefore, according to neither method, the multi-fiber ribbon cannot be divided into individual optical fibers efficiently by one step.
Especially, if the multi-fiber ribbon is a 12-fiber ribbon, it has a width of 3 mm, a thickness of 275 .mu.m, and an optical fiber pitch of 250 .mu.m. Then, if the precision is low, the optical fibers can be damaged.
Therefore, a method free from the above-mentioned drawbacks and capable of reliably separating only a specific optical fiber at high precision is sought.